asterisk users - Nov 2003 - SIP<->*/ISDN<->PSTN calls almost impossible because of echo

Hi,

I'm running an asterisk server using passive ISDN cards to connect to
the PSTN. SIP to SIP calls on the local network are ok, but
unfortunately the echo on outgoing calls (from a software SIP phone to
PSTN) is horrible: you'll hear the echo coming back after about 3
seconds. I've been searching the mailing list archives for 'echo'
and
found out that (a little bit of?) echo is unavoidable when working with
ISDN cards, but it seems to me that 3 seconds is just to long and that
something else must be wrong. Does anyone have an idea what might be
causing this extreme delay and/or how to research this problem? 

Server configuration: 

-P4 2.4Ghz, 
-512Mb, 
-40Gb HDD, 
-Two passive 'Etech' ISDN cards (+ i4l/Hisax) 
-Slackware 9.1

Clients: softphones, both SJPhone and Sipps

Thanks in advance,
Aart